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More on the Fiber-Positioner Hydra (1Jun92) (from KPNO, NOAO Newsletter No. 30, 1 June 1992) Multi-object spectroscopy at the 4-m is now carried out with Hydra, a fiber positioner which feeds light from the R- C focus into 97 blue or red fibers, which then carry the light down to the Bench Spectrograph in the large coud room. Hydra saw its first major use by visitors during a long observing block in February and March. These runs were quite successful, barring a few glitches and poor weather. The blue fiber cable was completed and installed for use during that observing block. The following throughput estimates were made for the blue cable under moderate seeing of about 1.3 arcsec: 0.9% at 3500 AŹ with grating KPC-10B 2.7% at 4000 A with grating KPC-10B 2.9% at 4500 A with grating KPC-10B 2.7% at 5000 A with grating KPC-10B The detector used was the T2KB CCD. The numbers give the percentage of photons incident onto the primary mirror that were detected. Raw flux counts (photons/second/A) are shown in the figure below for the flux standard star Feige 66 (m[3200] = 9.7, m[3500] = 9.8, m[4000] = 9.9, m[5000] = 10.3). The dominant throughput losses in the blue are due to the atmosphere and CCD quantum efficiency, not the fiber! The blaze efficiency of the grating (KPC-10B) dominates the efficiency slope redward of 4000 A. Potential users should realize that the efficiencies achieved in normal operations could be lower than the values cited above under conditions of poor seeing, in the event of astrometry errors, or at high airmasses where exact modeling of differential refraction is uncertain. [figure not included] The following figure shows the transmission curves for the blue and red fiber cables. Note that the blue cable can be used for observations out to 7000 A (and even out to 8500 A, if the 7200 A dip is acceptable). The quality of the prism mountings onto the fibers is better for the blue cable than for the red; this results in smaller fiber-to-fiber variations in throughput for the blue cable than for the red. The red cable also contains about a dozen fibers with very poor transmission blueward of 4500 A. It is therefore suggested that the blue fiber cable is the optimum choice for any observations blueward of 7000 A and that the red really only be used for observations redward of 7000 or 8500 A. Each cable contains 97 fibers. [figure not included] This observing block also saw the first use of the Bench Spectrograph automation hardware/software. Users were able to view the fiber ends and back-illuminate the fibers remotely via a program running on Khaki, the 4-m data acquisition computer. A second major observing block is scheduled for May and June. The internal comparison sources will be in use by that time. These lamps, used to illuminate a screen that is positioned by the gripper over the fibers, will improve the ease with which users can obtain wavelength calibration. Further improvements in the Hydra software are also underway, and more functionality is being added to the Bench Spectrograph automation hardware/ software. Further details of the instrument, and greater discussion of throughput issues, can be found in the extensive instrument manual. Copies of the manual can be obtained through the observing support office or through anonymous ftp. For access via the computer, ftp to orion.tuc.noao.edu (140.252.1.22), login as anonymous using your last name and machine for the password. Change directories with cd kpno/hydra, change transfer type with binary, and get the manual with get hydramanual.ps.Z which is in a compressed format. Uncompress the file with uncompress hydramanual.ps.Z and print it out with lpr -s -PlwN hydramanual.ps. The manual is on the order of 70 pages long and has several figures in it which will take some time to print out. Sam Barden, Taft Armandroff
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